parse_config_key: allow matching single-level config

The parse_config_key() function was introduced to make it easier to match "section.subsection.key" variables. It also handles the simpler "section.key", and the caller is responsible for distinguishing the two from its out-parameters. Most callers who _only_ want "section.key" would just use a strcmp(var, "section.key"), since there is no parsing required. However, they may still use parse_config_key() if their "section" variable isn't a constant (an example of this is in parse_hide_refs_config). Using the parse_config_key is a bit clunky, though: const char *subsection; int subsection_len; const char *key; if (!parse_config_key(var, section, &subsection, &subsection_len, &key) && !subsection) { /* matched! */ } Instead, let's treat a NULL subsection as an indication that the caller does not expect one. That lets us write: const char *key; if (!parse_config_key(var, section, NULL, NULL, &key)) { /* matched! */ } Existing callers should be unaffected, as passing a NULL subsection would currently segfault. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>